This invention relates to phase lock loops and, more particularly, to a phase lock loop with an active lowpass filter including a differential amplifier with reference and error voltage input circuits in which variations track as a function of time and temperature.
Phase lock loops are generally employed in AFC, in following a received signal, and in recovery of a clock signal from an input data stream. Such loops are described in the book Phase Lock Techniques by F. M. Gardner, John Wiley & Sons, Inc., 1966. One type of pulse timing recovery circuit using a phase lock loop is described in U.S. Pat. No. 3,757,244, Phase Locked Loop With Non-Linear Filter Responsive To Error Signal Amplitude by A. J. Giger, Sept. 4, 1973. When a high degree of accuracy is required, the loop may employ an active lowpass loop filter since it can have a very high DC gain and correspondingly small phase error. The differential amplifier of such an active filter requires a reference voltage about which it can integrate the output of the phase detector. The U.S. Pat. No. 3,775,695, Phase Lock Loop For A Voltage Controlled Oscillator by C. W. Hill, Nov. 27, 1973, describes a phase lock loop for use in following a target signal frequency. Hill's phase lock loop employs an active loop filter in which the loop error voltage from the phase detector is compared in an integrating amplifier with a fixed DC reference voltage for deriving the control voltage that drives a VCO. Hill's active integrator circuit and/or reference voltage are described in column 2, lines 36-46; column 3, lines 16-22; and column 3, line 63 to column 4, line 4 there. The Bell Telephone Laboratories' 500A type data service unit includes Exclusive-OR function phase detector means and an active filter in a phase lock loop that is employed to recover a clock timing signal from an input data stream. Bell's reference voltage is also a fixed DC value that is produced in a voltage divider circuit. When such a technique is employed, fine tuning adjustments such as selecting a resistor or trimming a potentiometer is required for each unit that is produced. Such a procedure is time consuming. Also, variations in circuit elements and parameters such as are caused by age and changes in temperature will normally produce phase errors in the output of the VCO.
An object of this invention is the provision of an improved phase lock loop which automatically compensates for changes in circuit elements and parameters in the input lines of a differential amplifier of the loop.